Image-forming MR methods, which utilize the interaction between magnetic field and nuclear spins in order to form two-dimensional or three-dimensional images are widely used nowadays, notably in the field of medical diagnostics, because for the imaging of soft tissue they are superior to other imaging methods in many respects, they do not require ionizing radiation, and they are usually not invasive.
According to the MR method in general, the body of a patient or in general an object to be examined is arranged in a strong, uniform magnetic field B0 whose direction at the same time defines an axis, normally the z-axis, of the coordinate system on which the measurement is based.
The magnetic field produces different energy levels for the individual nuclear spins in dependence on the applied magnetic field strength which spins can be excited (spin resonance) by application of an alternating electromagnetic field (RF field) of defined frequency, the so called Larmor frequency or MR frequency. From a macroscopic point of view the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse) while the magnetic field extends perpendicularly to the z-axis, so that the magnetization performs a precessional motion about the z-axis.
Any variation of the magnetization can be detected by means of receiving RF antennas, which are arranged and oriented within an examination volume of the MR device in such a manner that the variation of the magnetization is measured in the direction perpendicularly to the z-axis.
In order to realize spatial resolution in the body, constant magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field, leading to a linear spatial dependency of the spin resonance frequency. The signal picked up in the receiving antennas then contains components of different frequencies which can be associated with different locations in the body.
The signal data obtained via the receiving antennas corresponds to the spatial frequency domain and is called k-space data. The k-space data usually includes multiple lines acquired with different phase encoding. Each line is digitized by collecting a number of samples. A set of samples of k-space data is converted to an MR image, e.g. by means of Fourier transformation.
One special application of magnetic resonance imaging (MRI) is the assessment of vascular permeability and tissue perfusion, blood volume and flow, respectively. A way to obtain this information is the performance of dynamic contrast enhanced MRI (DCE-MRI) and dynamic susceptibility enhanced MRI (DSE-MRI). DCE-MRI delivers information about vascular permeability, the extravascular extracellular space, and perfusion. DSE-MRI provides feedback about blood volume and flow. Here, physiologic markers are derived by pharmacokinetic modeling of the concentration of a contrast agent over time in tissue and blood after injection. Dynamic changes of the concentration determined by the dynamic change of the longitudinal relaxation rate R1 (DCE) or R2* (DSE), induced by the injection of paramagnetic contrast agents (usually gadolinium (Gd)). DCE mostly uses steady state 3D spoiled gradient echo MR sequences and short repetition and echo times (TR, TE) to quantify R1 from changes in signal intensity. However, the impact of changes in susceptibility (R2* effect) on the signal intensity are neglected to simplify the approach, which results in inaccuracies. On the other hand, DSE normally employs single shot EPI spin echo or gradient echo sequences and long TR, TE to quantify R2*, and the paramagnetic R1 effect of the agent is neglected.
It has to be noted here, that R2* denotes the transversal relaxation rate of the spin system which includes contributions due to magnetic field inhomogeneity and R1 denotes the longitudinal relaxation rate of the spin system. Throughout the description, ‘relaxation behavior of the spin system’ is understood as the respective relaxation rate or relaxation time which is the inverse of the relaxation rate.
Dynamic oxygen or carbon dioxide enhanced MRI (D(C)O2E-MRI) is currently achieving increasing interest for the assessment of tissue oxygenation and vasoreactivity. These are important parameters for the selection of cancer treatments. E.g. the efficiency of radiation therapies depends on the oxygenation level of tumors.
The technique normally applies (multi-)gradient echo sequences for R2* quantification during oxygen or carbon dioxide breathing. Changes of R2* reflect changes in blood oxygenation or blood flow and volume, respectively. Concurrent changes of R1, induced by dissolved oxygen and flow, are physiologically interesting but are far lower in amplitude and difficult to measure (time consuming) and are thus mostly left unconsidered.
The simultaneous measurement of R1 and R2* in those dynamic approaches bears the potential to either improve accuracy (DCE) or specificity (concurrent physiologic measurement) of the physiologic output.